1. Field of the Invention
The present invention generally relates to a method for aligning wafer, and more specifically to a method for aligning wafer wherein a non-defective wafer alignment mark which is used as a reference image is compared to wafer alignment marks and a defective wafer alignment mark image is replaced with the reference image to obtain alignment information of an underlying layer, thereby improving the overlay accuracy for an underlying layer.
2. Description of the Related Art
In accordance with a conventional wafer alignment method, alignment information such as an offset, a wafer scale, orthogonality and wafer rotation to an underlying layer are calculated from a plurality of arbitrary wafer alignment marks on each die of a wafer, i.e. from alignment marks selected from a job file by a wafer alignment sensor in an exposure process.
In accordance with the conventional method for aligning wafer, even when only one of the selected alignment marks has a defect, the overlay accuracy of a lithography process may be degraded severely. As a result, detection of alignment signal by the wafer alignment sensor was not possible.
In addition, as long as the calculated alignment information for the underlying layer, such as the offset, the wafer scale, the orthogonality and the wafer rotation does not exceed the limit set in the exposure apparatus, the measured wafer alignment value was not reflected n wafer alignment even though the value was sufficient to affect the alignment accuracy. As a result, the overlay accuracy was deteriorated.